Photodiodes are widely used for sensing light and infrared radiation. The signal-to-noise ratio which can be obtained from photodiodes is limited by the level of thermal noise, which in turn is related to the temperature of the component. The term “dark current” is commonly used in the art to define the current flowing in a photodiode during a totally dark condition. The signal-to-noise ratio in photodiodes is conventionally improved by cooling the component, usually down to a temperature which can be maintained by a liquid nitrogen coolant (77K). The means for cooling the photodiodes down to a low temperature and keeping them there are cumbersome and expensive. It accordingly has been a long-term objective to provide photodetectors which can operate closer to room temperature but at acceptable signal-to-noise ratios. The noisiness of a detector can be quantified by calculating its noise equivalent temperature difference, which is the minimum signal derived from the temperature difference between a target and its background that yields a signal-to-noise ratio of unity. Minimization of the NETD is desirable.
Detector cells conventionally accumulate charge in respective integration capacitors. A typical well depth for an integrating capacitor is about 107 electrons. In infrared detector applications it will typically be filled by background radiation in about 1 ms. Large dark and radiation background currents more quickly saturate the integrating capacitor. If the dark and background current in such devices can be eliminated or reduced, their signal-to-noise ratios will be improved, and the NETDs will be reduced, permitting longer integration times. Further, the size and area of the integration capacitor can then be reduced, decreasing overall pixel size and increasing resolution of an array of such devices, and dynamic range can be enhanced.